Ethylene/α-olefin rubbers, such as an ethylene/propylene copolymer rubber (EPR) and an ethylene/propylene/diene monomer copolymer rubber (EPDM), typically have no unsaturated bond in the main chain of their molecular structures. EPR and EPDM are thus excellent in heat aging resistance, weathering resistance and ozone resistance as compared with general-purpose conjugated diene rubbers, and they have been broadly used for automobile parts, electric wire materials, electric/electronic parts, building and civil engineering materials, or industrial material parts.
In recent years, the need for lightening and lengthening of life of various parts has increased, thus higher performance properties have been strongly desired. Moreover, quality control standards for many molded or extruded products are becoming more stringent giving rise to more demanding material design to remove production and extrusion problems.
In EPDM prepared by the use of a conventional Ziegler Natta catalyst, there is a fear that the residual metal component derived from the catalyst causes deterioration of heat resistance, occurrence of foreign matter, or inhibition of vulcanization reaction. Further, there is a problem of difficulty in control of molecular structure of the copolymer because the catalytic activity during copolymerization is low.
In order to enhance production processability and extrusion processability of EPDM, it is desirable for the polymer viscosity to be moderately low. However, if the viscosity of the polymer is too low, sealing properties and strength of the resulting crosslinked rubber article are negatively impacted making the crosslinked rubber article less useful. Widening molecular weight distribution and/or composition distribution to decrease the viscosity of the polymer is practiced, but there often occurs problems that the low-molecular weight material causes fogging of window glass in automobiles or rooms, tackiness on the article surface, and brittleness at low temperatures.
One method to improve retention of shape in the extrusion of EPDM is to increase the viscosity of the rubber compound, but if the viscosity is increased, the extrusion rate is typically reduced, resulting in a problem of poor surface texture of the extrudate, among other things.
WO 2000/59962 tried to improve sealing properties and strength of a vulcanized rubber article obtained from EPDM with maintaining kneading processability and extrusion processability; however, the results were not satisfactory from the viewpoint of a balance between processabilities and sealing properties or strength.
U.S. Pat. No. 6,686,419 discloses multimodal ethylene/alpha-olefin/diene polymers and processes for making such using multiple reactors in series or parallel. Specifically Examples 1-8 disclose polymerizations of ethylene/propylene/ethylidene norbornene using bis(p-triethylsilyl-phenyl)methylene(cyclopentadienyl)(2,7-di-tert-butyl-9-fluorenyl) hafnium dimethyl combined with N,N-dimethylanilimium(tetrakis pentaflourophenyl borate). According to column 13, line 23, et seq. property control of the first and second fractions was achieved by adjusting the hydrogen to each reactor, as well as varying the monomer and catalyst feed rates.
U.S. Pat. No. 6,506,857 discloses Group 4 organometallic compounds comprising two ancillary monanionic ligands, each of which independently may be substituted or unsubstituted, wherein the ligands are bonded by a covalent bridge containing a substituted single Group 14 atom, the substitution on said Group 14 atom comprising aryl groups at least one of which contains at least one hydrocarbylsilyl substituent group, such as bis(p-trimethylsilyl-phenyl)methylene(cyclopentadienyl)(2,7-di-tert-butyl-9-fluorenyl) hafnium dimethyl.
U.S. Pat. No. 6,255,427 discloses catalyst combined with a first cocatalyst of alumoxane and a second cocatalyst of perfluoroarylboranes.
US 2012/0316302 discloses catalyst systems comprising multiple non-coordinating anion activators and methods for production of crystalline polymers therewith.
Others have mentioned use of alumoxane or scavenger or aluminum alkyl and non-coordinating anion activators for production of polymers such as EPDM or EPR, such as U.S. Pat. No. 6,034,022, U.S. Pat. No. 6,420,507, U.S. Pat. No. 5,948,873, WO 2012/130922, and EP 0 708 117.
Another method of increasing processability is to produce bimodal elastomers having a major fraction of lower molecular weight polymer (Mooney viscosity less than about 100) and a minor fraction of high molecular weight polymer (Mooney viscosity greater than 120), as in ExxonMobil Chemical's Vistalon™ Bimodal EPDM grades. However, these polymers tend to have an elasticity which is lower than desired for some applications. There is still a need in the art for new and improved methods of producing processable EPDM polymers having good elasticity, preferably in a tunable continuous process.
Other referenced of interest include: U.S. Pat. No. 5,698,651; EP 2 115 018; U.S. Pat. No. 7,829,645; U.S. Pat. No. 7,956,140; EP 0 751 156; U.S. Pat. No. 8,058,373; U.S. Pat. No. 8,101,696; U.S. Pat. No. 8,013,082; WO 1997/032946; WO 2011/002199; KR 2011/059405; US 2011/065877; WO 2010/012587; U.S. Pat. No. 7,989,565; U.S. Pat. No. 3,674,754; U.S. Pat. No. 4,510,303; US 2004/0220359; WO 1998/49212; US 2009/0209672; BR 2005/001339; U.S. Pat. No. 7,148,305; U.S. Pat. No. 6,924,342; U.S. Pat. No. 6,657,027; U.S. Pat. No. 7,169,865; U.S. Pat. No. 7,300,983; U.S. Pat. No. 7,271,221; U.S. Pat. No. 6,875,816; JP 2000/256423; U.S. Pat. No. 6,410,650; U.S. Pat. No. 6,420,507; U.S. Pat. No. 6,680,361; WO 1998/006728; U.S. Pat. No. 5,977,251; EP 784 062; U.S. Pat. No. 6,723,794; U.S. Pat. No. 5,798,427; U.S. Pat. No. 5,902,867; U.S. Pat. No. 6,348,555; EP 2 221 323; U.S. Pat. No. 7,135,533; and WO 2001/85839.
See also, Macromolecules, 1968, 1, 358; Macromolecules, 2001, 34, 3115; J Rheology, 2002, 46, 401; and J. Matls Proc Tech 2009, 209, 520.
There is still a need in the art for new and improved methods of producing processable EPDM polymers having good elasticity, preferably in a tunable continuous process.